The Story of the Truth Window


When you’re new to straw bale building, one of the first things you may notice or hear about is the “truth window.”

What exactly is this feature? It seems to me like a fun little joke within the community of straw bale builders and/or homeowners, where a small part of the wall (interior or exterior) is left without plaster, so that the straw is visible. This acts as proof to unbelieving guests and visitors that the walls are actually insulated with straw bales.

The truth window opening is covered with non-removable glass, left open, or has a door that opens and closes to allow the straw to be both seen and touched. However, we STRONGLY recommend sealing this section of the wall and detailing it well to prevent air and moisture from moving through the walls via this penetration. A simple pane of glass embedded in the plaster should do the trick.

Designing a truth window is a fun process where you can express your style and individuality by customizing the trim, shape, or other features. A quick online image search can give you inspiration, or check out a few we’ve worked on and around.

DeGarmo Strawbale Residence

This beautiful contemporary southwest style home, built by Black Timber Builders, features straw bale insulated walls, interior clay plasters, and tadelakt showers, sink, and kitchen backsplash by Living Craft.

Case Study: A Straw-Cell Addition

straw wallsLiving Craft Design has been working on a straw bale addition to our friends’ home in Boulder. This family is already well-versed in the benefits of natural building, so their addition incorporates passive solar heating with large, south-facing windows and a concrete slab floor to hold all that sun-generated heat. The addition has the high insulation value (R-value) that is a great feature of straw bales as a building material. Additionally, the family chose to renovate the original rooms of their home and filled the walls and ceilings with densely-packed blown cellulose as part of that process. This home will be comfortable year-round and have low energy bills as well as a reduced need for running heating and cooling systems.

 

A Straw-Cell Design

We jumped into the project starting with straw bale installation. This house has what’s called a straw-cell wall, with an entire wall system behind the bales, complete with wood studs, recycled denim insulation, and exterior wood siding. The bales then are stacked on the inside of this wall, meaning that labor is reduced because you don’t have to cut or trim as many bales to fit within and around the walls’ wood frames. You also only need to apply plaster to the interior side of the bales.

Low Carbon

A great thing about the insulation materials used in this home is that they are all carbon-based, and will now be locked away in the walls of this home for a very long time. This is actually a form of carbon sequestration, one technique to keep atmospheric carbon down and help mitigate climate change. This is in direct contrast to insulation materials like foam, fiberglass, and mineral wool, which consume a lot of energy to create. This means that the manufacture of those materials produces a lot of greenhouse gas emissions, upping their carbon footprint.

Healthy, Natural Plasters

Applying a base layer of clay plaster at the work party.
Applying a base layer of clay plaster at the work party.

Our next step was to start the plasters. A base coat of local red clay was applied to cover the bales first. At a fun summertime work party hosted by the family, we added a leveling coat which will provide the base and shape for the finish plasters. We introduced some folks to the techniques and tools for mixing and applying natural clay plasters and played in the mud with good friends. We also started to build out the windowsills into their final shape using a lime and clay mixture with lots of straw for strength.

Ben talks building with some volunteers while others work on shaping the windowsills.
Ben talks building with some volunteers while others work on shaping the windowsills.
The finish plaster drying around the laundry room window.
The finish plaster drying around the laundry room window.

The final wall finish was customized to get the exact color and texture that our clients wanted, and this last, thin layer of clay-based plaster went on like a dream. Thanks to additions like wheat paste, it dries into a hard and durable finish that will last for years, and also be easily reparable in case of accidental damages.

During the wall finish process, we were preparing for and creating the tadelakt windowsills that will become perfect benches for sitting on and reading in the natural light. This Moroccan finish plaster is created from lime, which is why the base coat for the sills incorporated lime with the clay. This results in a tight bond between the materials. The smooth layers of lime were applied, soaped, and burnished using stones to create a shiny finish. The sills were then waxed as the final step to create a long-lasting surface that can withstand some use.

The burnished and waxed tadelakt windowsills have an amazing shine and reflection, as well as feeling silky smooth.
The burnished and waxed tadelakt windowsills have an amazing shine and reflection, as well as feeling silky smooth.

The End Result: A Beyond Green Building

We’re departing this project with lots of hope for the future which will unfold in this family’s happy, healthy, non-toxic, and environmentally friendly home. Not only will it be a beautiful space to raise a family, but also a good model for other front range homeowners looking to sustainably add some space and renew their original home.

finished window and wall plaster

Case Study: Repairing a Strawbale Home

after-sponge-coat
The home after we applied a thin lime plaster over all the repairs.

Our clients for this project had finished building their own straw-bale home in the mountains west of Boulder about a decade ago. However, the harsh mountain climate and some details of the design and material selection had led to failures in areas of plaster, particularly around windows and in places where the roof drainage lacked adequate gutter systems.

lime-stabilized
The lime-stabilized earth layer which was used to repair damaged areas of the home, seen as the darker, lower level in this picture, has been built out by the homeowner to the same level as the original lime plaster. Now it’s ready to get a thin lime plaster over them all, to blend the different sections of the home together.

We worked with them to come up with a solution that would allow them to participate in the repair work, while using our experience and tools to speed up the process and get the home ready for winter.

First steps were to remove the lime plaster where it had begun to delaminate from the clay plaster base coat below. This happens because of a reaction between the two materials that creates a silt layer which prevents them from bonding, weakening the walls. In some areas, water had eroded the clay base coats, and we needed to mix and apply a clay plaster to build out everything to the same level. Next, we mixed up an intermediate lime-stabilized earth plaster to apply on top of the clay base coat. This in-between layer helps bond the two materials by creating a transition. The homeowners applied both of these layers by hand after we mixed up several batches and staged them around the house. They also took the opportunity to enhance and add some sculptural elements, including the house number in relief on the side of the wall.strawbale-home-number

We also added metal flashing around windows and below siding to move any water running off the building out to the surface of the wall and keep it from infiltrating into the plaster again. A final sponge-finished lime plaster over the whole house covers up any differences between the old and new and seals boundaries to prevent water from eroding the surface.

strawbale-repairs-in-progress

We will be able to go back in the spring next year, once temperatures are consistently above freezing, to apply a final colored coat over their home. This entire process will ensure that the will last for many decades to come, even in the harsh mountain winters.